1. Field of the Invention
The present invention relates to an inverter apparatus for supplying a driving voltage to a fluorescent lamp, and more particularly to, an inverter apparatus used for a power supply apparatus including a primary region and a secondary region which need to be electrically insulated from each other.
2. Description of the Related Art
Recently, as a substitute for CRT televisions, thin, wide screen liquid crystal televisions have been widely provided. As a backlight for the liquid crystal television, a plurality of cold cathode fluorescent lamps (hereinafter, referred to as CCFLs) or a plurality of external electrode fluorescent lamps (hereinafter, referred to as EEFLs) are disposed on a rear surface of a liquid crystal panel.
The liquid crystal television includes a power supply apparatus which performs AC/DC conversion of a commercial AC voltage to a DC voltage (hundreds of volts). The power supply apparatus is divided into a primary region and a secondary region which need to be electrically insulated from each other.
Now, an inverter (DC/AC converter) which cooperates with the power supply apparatus to convert the DC voltage of hundreds of volts to a boosted AC voltage and supply the AC voltage to the fluorescent lamp is considered. FIG. 1 is a block diagram illustrating a construction of a power supply system including the primary and secondary regions. The power supply system 500 includes an AC/DC converter 410 which converts the commercial AC voltage Vac to a DC voltage Vdc and an inverter apparatus 420 which converts the DC voltage Vdc output from the AC/DC converter 410 to an AC driving voltage Vdrv and supply the driving voltage Vdrv to the fluorescent lamp 430 which is a load.
For example, the AC/DC converter 410 may include a full-wave rectifier circuit 412 and a power factor control circuit 414. A DC voltage Vdc which is an output voltage from the AC/DC converter 410 is output to the inverter apparatus 420.
The inverter apparatus 420 includes a control circuit 422 and a transformer 424. The control circuit 422 monitors an electrical condition of a current flowing through the fluorescent lamp 430 which is to be driven or a voltage applied thereto and performs feedback control of a switching voltage applied to a primary winding of the transformer 424.
Referring to FIG. 1, the power supply system 500 is divided to the primary region 510 and the secondary region 520. In the power supply system 500, the AC/DC converter 410 is disposed on the primary region 510, and the fluorescent lamp 430 is disposed on the secondary region 520. As shown in FIG. 1, apart of the inverter apparatus 420 is disposed on the primary region 510, and the another part thereof is disposed on the secondary region 520. More specifically, the control circuit 422 and the primary winding of the transformer 424 is disposed on the primary region 510, and the secondary winding of the transformer 424 connected to the fluorescent lamp 430 is disposed on the secondary region 520. The control circuit 422 needs to monitor the electrical condition of the fluorescent lamp 430. As described above, the primary region 510 and the secondary region 520 need to be electrically insulated from each other.
In the power supply system 500, a photo coupler 440 is used to feed the electrical condition of the fluorescent lamp 430 disposed on the secondary region 520 back to the control circuit 422 disposed on the primary region 510. For example, a current of the fluorescent lamp 430 is converted to an electrical signal Sfb1 by a resistor element 432. The electrical signal Sfb1 is input to the photo coupler 440, so that the electrical signal can be converted to an optical signal by a light emitting diode. The converted optical signal is received by a photo diode or a photo transistor disposed on the primary region 510 to be converted to an electrical signal Sfb2, and electrical signal Sfb2 is fed back to the control circuit 422. The related art is disclosed in Japanese Patent Application Laid-Open No. 2003-153529 (Patent Document 1). In some cases, the feedback of the signal from the secondary region 520 back to the primary region 510 by using electromagnetic coupling of the transformer instead of the photo coupler.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-153529
As described, in a case where the feedback of the signal from the secondary region 520 back to the primary region 510 is performed by using the photo coupler, since the electrical signal is converted to the optical signal, there is a problem in that accuracy of feedback deteriorates. In addition, if an efficiency of coupling between light emitting elements and light receiving elements varies over all the photo couplers, there is a problem in that luminance of a fluorescent tube varies.
In addition, since a plurality of the fluorescent lamps are used as the backlight of the liquid crystal television, the feedback of the signal from the secondary region 520 back to the primary region 510 needs to be performed through a plurality of paths. In this case, since the photo coupler needs to be provided to each path, there is a problem of high cost and increase in mounting area.